In the oil industry, electromagnetic (EM) induction surveys are used to map the electrical conductivity of geologic formations between boreholes and/or radially away from a single wellbore. The latter, usually referred to as induction logging, has been in routine use for over fifty years. Those surveys are performed in open holes; that is, holes that have not been lined with a (typically, metal) casing.
Recently, the concepts of induction logging have been extended to surveys between uncased wells and between wells cased with conductive liners. There is also interest in the use of logging between surface and downhole sensors, and within single wells that are cased with conductive liners.
Airborne electromagnetic surveying and surface based EM surveying are each individually known in the field of geophysics. Such airborne EM survey and surface based EM survey configurations are well described in “Transient and Frequency Electromagnetics” by Kaufman, in “Electromagnetic Methods in Geophysics” by Hohmann et al, 1991.
Airborne EM surveys to date have been limited to geometries and surveys to explore for metallic ores or mapping near surface conditions, such as environmental problems, and the limitations of airborne EM sources in the prior art have prevented success in applications for wellbore receiver technology for subsurface formation imaging of a hydrocarbon reservoir.
Surface to borehole EM (as well as borehole to surface configuration) surveys have been described in detail in applications commonly owned with the present application, including:                U.S. Patent Application Ser. No. 61/160,056 filed Mar. 13, 2009, entitled “Electromagnetic Detection of Base of Salt While Drilling”,        U.S. Patent Application Ser. No. 61/173,246 filed Apr. 28, 2009, entitled “Electromagnetic Detection of Base of Salt While Drilling”,        U.S. patent application Ser. No. 12/581,947 filed Oct. 20, 2009 entitled “Detecting Electrical Current in a Magnetic Structure”,        U.S. patent application Ser. No. 12/641,944 filed Dec. 18, 2009, entitled “Correction Factors For Electromagnetic Measurements Made through Conductive Material”,        U.S. patent application Ser. No. 12/641,898 filed Dec. 18, 2009, entitled “Attenuation of Electromagnetic Signals Passing Through Conductive Material”,        U.S. patent application Ser. No. 12/603,053 filed Oct. 21, 2009, entitled “Electromagnetic Logging Between Borehole and Surface”,        U.S. patent application Ser. No. 12/405,214 filed Mar. 16, 2009, entitled “Casing Correction in Non-magnetic Casing by the Measurement of the Impedance of a Transmitter or Receiver”        
The various types of induction surveys typically share many commonalities. A transmitter, usually a multi-turn coil of wire, carries an alternating current of frequency ω (radians/sec). This creates a time-varying magnetic field in the surrounding formation that in turn, by Faraday's law, induces an electromotive force (emf). This emf drives currents in the formation that are basically proportional to the formation conductivity. Finally, a receiver is positioned either in the same hole as the transmitter, in another hole, or on the surface (land or seafloor), and measures the magnetic field arising from the transmitter and the secondary or induced currents in the formation. Conventional induction logging always uses a combination of multiple receivers and/or multiple transmitters connected in series so as to cancel the mutual signal in air. In general, a theoretical model for a logging system embedded in a formation of arbitrary resistivity is used to match or interpret the received signals. In some applications, the absolute value of the average formation resistivity is not as important as the ability to map variations of resistivity within the formation. To determine this spatial variation of formation resistivity, the surveys typically involve placing the transmitter at multiple locations in the hole and measuring the fields at multiple receiver locations for each transmitter location. In crosshole surveys, this yields a data set similar to those obtained from tomography.